Test Overview

Vision is the result of electrical signals that travel between the retina and the part of the brain involved with vision (occipital cortex).

Electrophysiology tests check to see how well this visual nerve pathway sends the electrical signals needed for vision. These tests measure electrical activity that occurs in your eye when you look at something.

Electrophysiology includes different tests that measure how well the retina works. It can help check for diseases of the retina. The tests may also help diagnose and evaluate different kinds of vision and health problems.

Electroretinography (ERG)

ERG measures the retina's electrical response while you look at different patterns or flashes of light. An ERG test can check for diseases and problems of the retina.

A full-field ERG can check how well your entire retina is working. A multifocal ERG (mfERG) tests just part of the retina. It can check for diseases of the macula and for central vision loss.

Electro-oculography (EOG)

EOG tests how well electrical currents are working in the whole eye. It is done to check for certain eye and retina problems.

Visual evoked response (VER)

VER tests the electrical activity in the whole visual pathway, from the eyes to the parts of the brain involved with vision.

Like ERG, this test measures electrical activity when the eye responds to looking at something. A VER test can find problems by showing how brain waves respond to certain things you look at during the test.

Why It Is Done

The different electrophysiology tests are done for different reasons.

Electroretinography (ERG)

A full-field ERG measures how well rod and cone cells are working. These cells help you detect light and colour. This test also looks at other cells in the retina. It may be used to check for problems such as:

• Retina damage from drugs or other substances.
• Loss of central or peripheral vision from retina problems.
• Night blindness (trouble seeing in low light) or colour blindness.
• Eye problems from vascular diseases.

A full-field ERG may help find the cause of certain retina problems, such as vitamin A deficiency or metabolic disorders.

A multifocal ERG (mfERG) tests the electrical response in the central part of the retina. It may be used to help check for:

Electro-oculography (EOG)

An electro-oculogram is used to help diagnose problems of the retina. It may be used to help check for problems such as:

• Diseases of the retina such as Best's disease (congenital macular degeneration) and retinitis pigmentosa.
• Different types of macular dystrophy.

Visual evoked response (VER)

Visual evoked response (VER) measures how well the whole visual pathway between the eye and the brain is working. The test may be used to check for or evaluate conditions such as retina problems, optic nerve problems, and multiple sclerosis.

VER can be used to check vision problems in people who can't take other eye tests. This includes infants or patients who can't respond to or follow instructions.

How To Prepare

You don't need to do anything special to prepare for the tests. Follow any instructions your doctor gives you about what to do before your test.

For visual evoked response (VER):

• Follow your doctor's instructions about eating to avoid low blood sugar.
• Avoid caffeine within 12 hours of the test.
• Wash your hair well before the test.
• Don't wear any jewellery, such as earrings, near your head.

How It Is Done

The way each electrophysiology test is done varies.

Electroretinography (ERG)

• You sit or lie down during the test.
• Eye drops are used to numb the eye.
• A tiny electrode is put on the front of your eye (cornea) and on your face.
• You will look at a screen that shows flashes of light (flash ERG) or a pattern of images (pattern ERG). The test records the electrical response in your eye as you look at the lights and patterns. The eye's electrical activity is shown on a machine called an oscilloscope.
• The test is done in both light and dark. This is called dark adaptation. Different wavelengths of light are used during the test to check for problems with the eye's rod and cone cells.
• The total exam takes about an hour.

Electro-oculography (EOG)

During the test:

• You sit in an exam chair.
• Tiny electrodes are placed on the inside and outside corners of the eye. Another electrode is placed on your forehead.
• Electrical activity in your eye is recorded on a machine. Two recordings are taken.
  • After your eyes adjust to the dark, the eye's electrical response is measured while you move your eyes from side to side.
  • With the light on, electrical activity is measured again as you move your eyes from side to side (at the same angle).
• Total exam time is about 40 to 45 minutes.

Visual evoked response (VER)

A computer records changes in your brain waves while you look at patterns. Electrical signals are measured in the visual pathway, the area of the brain involved with vision (primary visual cortex).

During the test:

• You sit in front of a screen. A patch is placed on one eye.
• Electrodes are put on your scalp, near the areas of your brain involved with vision.
• For several minutes, you look at repeated patterns (checkerboard or dotted) that appear in quick flashes on a screen. The size or intensity of the patterns may be changed to test certain parts of your visual field.
• A computer records the response of your brain waves. The test measures the time of response (latency) and the brain's electrical activity.
• A patch is placed on your other eye, and the test is repeated.

How It Feels

These tests most often cause little or no discomfort. Tests include electroretinography (ERG), electro-oculography (EOG), and visual evoked response (VER). The electrode used for an ERG test may feel like having an eyelash stuck on your eye.

Risks

There are usually no risks from these tests. Tests include electroretinography (ERG), electro-oculography (EOG), and visual evoked response (VER). After an ERG test, avoid rubbing your eyes for at least an hour. Rubbing your eyes may scratch the front of your eye (cornea).

Results

Electroretinography (ERG)

The test measures the amplitude (height) of certain brain waves (A-waves and B-waves) to detect vision problems. The time it takes for the eye to respond to light stimulus is called latency. Possible results are:

Normal.

You have normal A-waves and B-waves and a normal latency response time.

Abnormal.

You have abnormal A-waves and B-waves or an abnormal latency response time.

Electro-oculography (EOG)

Test results use a number measurement called the Arden ratio. This is the ratio of the eye's maximum electrical activity in light to the minimum electrical activity in the dark.

Normal.

The Arden ratio is within a normal range.

Abnormal.

The Arden ratio is lower than the normal range.

Visual evoked response (VER)

Normal.

There is no delay in neural conduction in the brain's visual pathway. (Your results are compared with baseline norms for the lab where the test is given.)

Abnormal.

There is a delay between the eye's stimulation and the nerve's response. Abnormal results may be a sign of problems in the visual pathway between the eye and brain.